1. Field of the Invention
The present invention relates to a circuit for testing integrated circuits and a test method thereof, especially to a test decompressor and a test method thereof that convert original input data of test input into test vectors for testing a circuit under test (CUT) containing scan chains. The test data volume can be significantly reduced, typically by a factor of hundreds or even higher, and a certain testable fault coverage is maintained. The scan chains in the CUT are divided into a plurality of scan groups. The scan chains in each scan group share the same test data (vectors) and hence the test data volume is significantly reduced.
2. Description of Related Art
The cost for testing integrated circuits rises along with the increasing complexity of integrated circuits. The high test cost results from large volume of test data, time-consuming test procedure and the number of test inputs. In order to reduce test data volume, test application time and the number of test inputs required, a method for test data compression has been proposed. A test data decompressor is inserted between test inputs and all scan chains. Data input is converted into test vectors for testing circuits. The main concept of the test data compression is to compress all test vectors and reduce the occupied storage space.
Broadcast scan is a popular technique for test data compression. A plurality of scan chains share test data from a test input. However, some faults may become undetectable as some test vectors required for different scan chains may conflict with each other. Thus these test vectors cannot be compressed by broadcast scan, resulting in fault coverage loss. In order to reach the fault coverage required, the previous work based on broadcast scan adds a series of flip-flops between the test input and the scan chains for providing different data to the scan chains. One multiplexer is required to switch different data for each scan chain and transfer the test data required into that scan chain.
However, such method has several shortcomings. First a part of circuit is unable to maintain the original fault coverage owing to reduction in fault coverage. Moreover, the big multiplexer requirement caused by the need of connecting different test data to scan chains and increased inputs of each multiplexer will lead to increased area required by each multiplexer. At last, the number of multiplexers required is increased along with the increasing number of scan chains. This results in high area overhead.
Thus there is room for improvement and a need to provide a test decompressor and a test method thereof to overcome the shortcomings mentioned above.